The University Group Diabetes Program (UGDP) was undertaken for the purpose of evaluating the effect of hypoglycemic treatment on the rate of development of vascular complications in mild adult-onset diabetics. It is a multi-clinic study involving 12 Clinical Centers and a Coordinating Center. Patient recruitment in this study started in February 1961 and was completed in early 1966 after a total of 1,027 patients had been enrolled. Patients were randomly assigned to one of the following treatment groups: Insulin given in fixed dose, insulin given in variable dose to maintain blood glucose levels within a normal range, tolbutamide, phenformin, or placebo. All patients were placed on diets designed to achieve or to maintain normal body weight. Information obtained from the study has and will be used to evaluate the efficacy of treatments under study in preventing or delaying the onset and development of vascular complications in mild diabetics and to provide detailed information on the natural history of diabetes. The clinical phase of the study ended in August 1975; annual patient follow-up and data analysis are currently the prime objectives.